The proliferation of modern wireless communications devices, such as cell phones, smart phones, and tablet devices, has seen an attendant rise in demand for large multimedia data capabilities such as streaming video, TV, music, and internet access at the mobile device or user node, also referred to as the user equipment (UE). To support this ever increasing demand for higher data rates new communications techniques are being developed to take advantage of new capabilities incorporated in modern mobile devices and access nodes. These new capabilities include improvements such as placing multiple transmit and receive antennas at the access nodes and user equipment to increase spatial diversity through the use of multiple input multiple output (MIMO) techniques.
A user obtains wireless service by establishing a connection or radio link between their mobile device and an access node in the communication network. The connection or radio link may be considered established when the signals between the mobile device and the access node are of sufficient quality to transmit data with an acceptable or predetermined quality of service (QoS). For example one metric used to evaluate QoS is the block error rate (BLER). A network may set a fixed floor value, such as 10%, or may use a formula to adaptively adjust the acceptable BLER based on current network conditions. For purposes herein an access node is a collection of one or more antennas configured to provide service to a contiguous geographic area, and transmission and reception of the collection of antennas is controlled by a single physical layer mechanism. The antennas and physical layer mechanism may be collocated at a single location or base station or they may be distributed at a number of locations in order to provide adequate service throughout the desired coverage area. The physical layer may include a single processing device or it may include a number of processing devices distributed at various locations with all the processing devices communicatively coupled by a communication network to allow them to jointly coordinate transmission and reception of the collection of antennas. The area covered by an access node, where transmission conditions are good enough to maintain a connection between the mobile device and the access node, is referred to as the coverage area of the access node and may also be referred to as a cell.
In a typical wireless communication network a plurality of access nodes is distributed over a geographical area where the access nodes are positioned to provide mobile users, generally referred to herein as user nodes, with wireless communication services. This geographical area encompassing the combined coverage area of all the access nodes is referred to as the service area.
When an access node is servicing more than one user node, meaning the access node has multiple mobile devices or user nodes connected to it, the user nodes need to be able to separate information being sent to them from information being sent to other user nodes within the same cell. This can be done by separating the signals in time by sending information to each user node during a different time slot. Signals can also be separated using frequency, where information is sent to all the user nodes at the same time using a different frequency band for each user node. Diversity necessary to separate signals can also be obtained by spatial separation where signals are sent from antennas that a distance apart. Spatial separation of the antennas results in each user node experiencing a different complex gain from each antenna. This allows each user node to separate their signals using spatial filtering or other signal processing techniques. A group of user nodes that are all spatially separated enough to allow separation of their respective signals is referred to as a spatial multi-user group.
In a conventional cellular network, each access node services user nodes within its coverage area or cell. As a user node moves between cells, the user node is handed off from one access node to another access node. A user node may be assigned to a cell based on its geographical location or alternatively the assignment may be done based on signal strength. With this conventional approach, a user node at the edges of a cell may suffer from weak signals and inter-cell interference (ICI) resulting in a lower level of service. It is often the case that a user node in the cell edge region may be able to establish a viable radio connection with multiple access nodes servicing adjacent cells.
If signals between a user node in this cell edge region and multiple cell sites are coordinated, both the downlink (transmissions from the network to the user node) and uplink (transmissions from a user node to the network) can be significantly improved. This type of coordination of multiple access nodes is referred to as coordinated multipoint (CoMP) transmission/reception. CoMP can be as simple as interference avoidance or in more complex systems, multiple access nodes can transmit the same data to a single user node thereby improving transmission quality through spatial diversity.
For downlink transmission, two types of CoMP may be used; coordinated scheduling also referred to as Coordinated Beamforming (CBF); and Joint Processing/Joint Transmission (JP/JT). With coordinated scheduling or CBF, the transmission to a single user node is transmitted from the servicing access node, exactly as in the case of non-CoMP transmission. However, the scheduling, including any beamforming functionality, is dynamically coordinated between the access nodes in order to control and/or reduce the interference between different transmissions. For JP/JT, the data sent to a single user node is simultaneously transmitted from multiple antennas controlled by multiple access nodes servicing different cell areas. These multi-point transmissions are coordinated as if they were being sent by a single transmitter with geographically distributed antennas.
Groups of coordinated access nodes are referred to as CoMP sets or CoMP clusters and provide a potential for much higher performance than coordinated scheduling alone. However, this higher performance comes at the cost of increased backhaul requirements. Backhaul is a term used for communication between access nodes via a mechanism different than the radio link used by the user node.
Forming groups of CoMP clusters in large wireless network environments presents a number of problems. Computational resources available for scheduling in any given area being controlled are limited, so the size of the area controlled, or the control area, will also be limited. When the control area is small, such as a single cluster of access nodes, extensive interference can be introduced from adjacent control areas. There is also a problem of how to handle the edges of control areas, since users that remain near a control area edge for a prolonged time may experience reduced service.
Thus, there is a need for improved apparatus and methods for forming clusters of access nodes in order to provide improved communication services while keeping resource requirements low.